Method of and apparatus for detwinnning quartz crystals



Oct. 31, 1950 A. N. HOLDEN 2,527,635

METHOD OF AND APPARATUS FOR DETWINNING QUARTZ CRYSTALS Filed Jan. 9, 1948 ZSheets-Sheet 1 FIG.

FIG. 2

INVENTOR A. /V. HOLDEN A TTORNEV Oct. 31, 1950 A. N. HOLDEN mz'mon OF AND APPARATUS FOR DETWINNING QUARTZ CRYSTALS Filed Jan. 9, 1948 2 Sheets-Sheet 2 INVENTOR A. N. HOLDEN ATTORNEY Patented Oct. 31, 1950 METHOD OF AND APPARATUS FOR DETWINNING QUARTZ CRYSTALS Alan N. Holden, New Vernon, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application January 9, 1948, Serial No. 1,457

13 Claims.

This invention relates to the producton of piezoelectric crystals, and particularly to means for converting electrically twinned quartz crystals to a uniform structure.

The employment of piezoelectric crystals for frequency control in oscillators, as elements of band-pass filters, and for a variety of related purposes has become of increasing importance. Those made from quartz are most satisfactory and have been the most widely used, although a few other materials and certain synthetic crystals have been found to give good results. The usable varieties of quartz are available in nature only in very limited quantities. Over half the quartz from which crystals have been out has had to be discarded because of the presence of twinning, since uniform orientation of the crystalline material is necessary to produce a satisfactory plate. Hence it is of considerable importance to be able to change twinned portions to a uniform crystalline structure.

Twinning is the simultaneous existence, within a single piece of the quartz, of regions of different orientation which might be regarded as derivable each from the other by a single step, such as rotation through 180 about a line through the crystal, or reflection through a plane in the crystal. A number of different types of twinning exist, but only two are of major importance and considered here.

Where the twinning is of the Dauphin type, which will be defined below, it may be corrected by the apparatus and method constituting the present invention.

This conversion can be accomplished by applying heavy homogeneous unidirectional stress and a strong constant potential in proper directions to the crystal blank after it has been heated to a temperature in excess of the inversion point between high and low quartz, and then allowing it to cool below that point, as will be described more fully hereafter.

A brief consideration of the crystalline structure of quartz will assist in understanding the invention. Quartz is made up of interlocking helices of silica tetrahedra. The helix axes are parallel to the optic axis of the quartz (C or Z). In high quartz, a form which occurs only about 573.3 C., a two-fold axis of symmetry of each tetrahedron extends parallel to the optic axis. In low quartz, which exists only below 573.3" C., the two-fold symmetry axes of the tetrahedra make an angle of 18 26' with the optic axis.

The divergences of the axes of symmetry from a position parallel to the optic axis may be such that successive tetrahedra are twisted like a right-handed screw, or like a left-handed screw. The direction of twisting is referred to as the sense of the helix. Parts of opposite sense are thus related as by mirror reflection. When parts of opposite sense are found in the same piece of quartz, twinning of the Brazil type is saidto be present. This arrangement is also known as optical twinning, since it is readily detachable by optical means.

The other type occurring commonly in quartz, known as Dauphin twinning, occurs when the tetrahedra are oppositely tilted from the helix axis in the two twinned portions in such a direction that the helices are related as by rotation about the optic axis through expressed in another way, the tetrahedra may be tilted by the characteristic 18 26 to the helix axis in eitherof two ways. Hence conversion of one type to the other could be effected if it were possible to shift the tetrahedra in one by the difference in tilt, or twice 18 26, and the present invention provides the method and apparatus for so doing.

In high quartz, as in low quartz, the senseof the helices may be either right or left. When right-handed low quartz is heated above 573.3 C., it becomes right-handed high quartz, in which the tetrahedra are no longer tilted, but the helices are still of the same sense. Therefore, Brazil twinning could not be cured by heat treatment together with the other steps here set forth. In the present state of the art no method is known for detwinning this type of quartz.

In contrast, Dauphin twinning is not present in high quartz. When such quartz is cooled below 573.3 C., it may assume either of the low quartz structures, and if means were available to prevent the assumption of more than one form, the formation of this type of twinning could be overcome. I

However, in the absence of some procedure such as that of which the subject-matter of the present invention forms an example, the high quartz, in one-quarter to one-half the cases, reverts on cooling to the identical low quartz struc ture it possessed prior to heating. In the remaining cases, the twinning reoccurs but in dif-l ferent arrangements than before.

While the reasons for the occurrence of twinnign are not well understood, it is believed that the reoccurrence of exactly similar crystal structheir original growth as low quartz. These defects would be present in such a way as to predispose the crystal to assume one or the other of the possible crystal structures, and would hence be similarly effective each time the crystal was cooled through the conversion point from high to low quartz.

When such predisposing factors are not present, it may be that the transitional structure at the composition plane, though at higher energy than the ideal lattice, may be of lower free energy under some transient condition of growth, and this structure would then be frozen in because of the high activation energy required for any later reversion to the ideal lattice.

Whatever the exact reason for the observed behavior, it has been found that quartz can be detwinned by allowing it to cool from the high temperature form under the combined application of a mechanical stress and an electric field, provided that the stress and field be in such directions that the scalar product of field and piezoelectric polarization have different values in the twin portions of .opposite senses; and that the electrostatic stored energy difference, represented by the difference in scalar products, be larger than any countervailing energy difference, either in elastically stored energy or electrostatic energy allocable to polarization components induced by the field itself.

These conditions are clearly met in X-cu plates for a homogeneous unilateral compressive stress and an electric field applied through the thickness of the plate.

The details of the method and apparatus which form the subject-matter of the invention may be better understood by reference to the drawings in which:

Fig. lis in elevational view of apparatus arranged for practicing the method;

Fig. 2 is a schematic sectional view of the chamber for treating the quartz;

Fig. 3 is an enlarged view of a slab of crystalline quartz in which twinning is present; and

Fig. 4 shows comparatively the numbers of crystal plates that may be cut from the quartz shown in Fig.3 before and after detwinning.

It is to be understood that the exact form of the apparatus shown is exemplary only of the principles of the invention, and that it is susi=2 ceptible of modification and to the use of equivalent structures and methods within the scope of the appended claims.

In Fig. 1 is shown the equipment for practicing the invention, incorporating a conventional hydraulic press to apply the desired pressure. Mounting pedestal carries the press table 2, from which rise columns 4 holding the top block 5. -Within the top block 5 is disposed the working cylinder 6, which actuates the plunger 1. A handwheel 8 is provided to retract plunger 1 when desired. Hydraulic pressure is applied to the working cylinder. 5 through a line 9, the pressure being indicated by a gauge If The system for producing the pressure is shown conventionally as a tank II with a hand pump i2, but it will be obvious that any-system for supplying liquid to the working cylinder 6 under adequate pressure would be the full equivalent of the means shown.

The plunger 1 applies the pressure from cylinder 6 to the crystal, which is disposed within the temperature-controlled chamber M. The chamber |4= may be formed by a cylindrical section 5 of heat insulating material, closed at the top by g. to each other.

an overlappingly seated cover l5. Plunger 1 passes slidably through a collar H in cover I6. It then is joined to a pressure plate l9 by conventional means. Pressure plate |9 is many times greater in cross-sectional area perpendicw lar to the axis of plunger 1 than the lunger itself. The unit pressure is thus reduced to a value consistent with the successful interposition of a sheet 29 of insulating material of high mechanical strength. Such a sheet be made of transite, a product formed of closely bonded asbestos fibers and a good insulator of both heat and electricity.

Beneath sheet 20 is fastened a second pressure plate 2| to which is fixed an auxiliary plunger 22 of heat-resisting metal, the lower end 24 of which is carefully finished to provide a fiat pressure surface exactly normal to the direction of travel of the plunger 22. The plunger 22 is bored near its lower end 24 to receive a thermocouple 25. Leads 26 passing through a suitable bushing 21 in the pressure plate 2|, sheet 20, and pressure plate i9, and through a bushing 29 in cover i6, provide connection to a meter 30 which may be calibrated to indicate the temperature adjacent the crystal slab or wafer 3|.

Crystal slab 3| is supported by a lower standard 32, having an anvil 33, exactly planar and normal to the direction in which force is to be applied, mounted on a large pressure plate 34 resting on an insulating plate 35 similar to transite plate 20, which in turn is held by the press table 2. This slab must be carefully made so that its major faces-are planar and parallel Thus uniform pressure per unit of area may be applied through the lower end 24 of plunger 22, and anvil 33 on lower standard 32.

This arrangement permits uniform heating and cooling of the crystal 3| and at the same time permits application of adequate pressures. As an aid in securing uniformity of pressure over the entire surface of crystal 3|, thin shims 35 may be placed above and below it. These shims may be of a malleabl metal such as soft platinum, which will flow sufficiently to equalize the applied pressure over the entire crystal face, if inequalities still exist in spite of the care used in manufacture.

It is also desirable to introducea fine layer of graphite as.a lubricant. This layer, not visible in the drawings, may cover the crystal faces in contact with the shims 36. Insome cases the shims may be omitted and only the lubricant used. The effectof the lubricant is to assist in minimizing'the deve1opment,-at the surfaces of the quartz plates, of large tangential stresses as the press and the crystals 0001. Such stresses develop because of the difference between the rates of thermal expansion and contraction in the quartz and in the plunger material, and may cause fracture of the blank'in some cases. In some applications it may be desirable to face the plunger 22 and lower standard 32 with'a material having a thermal coefficient of expansion equal as nearly as possible to that of quartz in order to minimize these stresses.

Heating the crystal to the desired temperature in excess of 5'73.3 C., the point of inversion from low to high quartz, is conveniently accomplished.

b introducing into the chamber M a ceramic tube 31 wound withresistance wire 39 such as a chrome-nickel alloy of high melting point. The temperature should be substantially in excess of the inversion point'in rder'thatall of the crys- 5. tals will be completely inverted to high quartz. Suitable means familiar to those skilled in the art may b employed to maintain the temperature at the desired value by controlling the flow of current through the heating wire 39, to which external connection is provided, through wall bushings 40, by means of leads 4|. Additional insulation is provided by means of annular asbestos sheets 42 and 44 above and below the ceramic tube 3'! and concentric about plunger 22 and lower standard 32. This arrangement of the heating chamber permits uniform heating for all parts of the crystal and facilitates gradual cooling without cracking the material. Gradual cooling may be obtained by progressively reducing the heating current until the chamber temperature reaches about 200 C., which may require a period of about two hours. The current may then be out off, and cooling to room temperature permitted by radiation, which mayrequire an additional period of the same order. I The pressure and potential may be removed at'the same time the heating current is cut off.

The steady potential to be applied to the crystal may be of the order of 10,000 volts per centimeter of thickness. It is introduced through plunger 22 and lower standard 32 by means of leads 45 and 46 entering through insulating wall bushings 41 and 49, respectively, and energized by a suitable direct current source, not shown in the figures. Lead 45 may be connected to plate 2| by set screw 48, while lead 46 may be connected to plate 34 by means such as set screw 50. The potential i insulated from the bod of the press by the non-conducting plates 20 and 35.

Thus the three factors necessary to accomplish the detwinning action are provided; the crystal heated well above the low-to-high quartz inversion point, high homogeneous unidirectional pressure applied normal to the-major faces of the crystal, and high steady potential applied between the faces while the crystal is cooled gradually through the inversion point. I l g A brief conslderation of the results actually obtained by the method may be had by reference to Figs. 3 and 4. A crystal blank- 59, about X x /64" in size, is illustrated in Fig. 3. Before treatment it carries the twinned areas irregularly throughout, as indicated by the unshaded area 50 denoting a Dauphin twinned portion of one sense, and the stippled portion 62 indicating a twinned area of the opposite sense.

The same quartz blank 59 is shown after the detwinning process in Fig. 4. It will be observed that the unstippled area 60 of one sense now covers substantially all of the crystal face. Minor areas of the opposite sense 62 were still observable along the periphery of the crystal. It is thought that this is probably due to inequalities in the pressures actually applied.

The gain in the useful quantity of the quartz is clearly seen by comparing the typical plates which may be cut from the blank 59 before and after treatment. Plates of 35 AT cut, about /8" square, were to be obtained. The plates are shown in outline, with the cross-hatched plate representing that which could be cut from the original twinned quartz plate, and the unshaded outlined areas H denoting those plates which would be available for cutting after the detwinning operation. It will be seen that in the particular specimen of Figs. 3 and 4, two plates could be out after treatment, whereas only one could be obtained before.

It is believed that about 50 per cent of the 6 quartz which must now be thrown away because of twinning can be made usable by this process. Thus the invention provides simple means for utilizing more eificiently the limited supplies of natural quartz crystals available for piezoelectric purposes.

What is claimed is:

l. The method of treating a quartz crystal wafer having parallel major faces and containing Dauphin twinning to produce a uniform crystalline structure therein, comprising the steps of gradually heating said crystal to a point in excess of the low-high inversion temperature, applying a homogeneous compressive stress normal to said faces, applying a steady potential difference to said faces, and simultaneously therewith cooling said wafer through the low-high quartz inversion temperature.

2. The method. of producing a crystal wafer of uniform crystalline structure from a quartz wafer having twinned areas of the Dauphin type, comprising heating said wafer about the low-to-high quartz inversion point, applying a potential across said crystal, applying a pressure distributed uniformly per unit of area to the major faces of said crystal, and cooling said wafer even-.- ly to a temperature below said inversion point While continuing to apply pressure and potential thereto.

3. The method of detwinning a quartz crystal wafer having substantially parallel major faces, and in which Dauphin twinning is present, which comprises heating said wafer gradually and uniformly to a point above the temperature of inversion from low quartz to high quartz, applying a constant potential between said major faces, applying a substantial pressure uniformly distributed over the normal to said major faces, and

cooling said wafer below the temperature of inversion from low quartz to high quartz while applying such constant potential and substantial pressure. 1

4. The method of treating a quartz crystal wafer having substantiall parallel major faces to remove Dauphin twinning, which comprises heating said wafer to a temperature above the between said major faces during such cooling,

6. The method of converting a quartz crystal slab in which Dauphin twinning is present to a uniform crystalline structure, comprising heating said slab above the inversion point, applying uniform mechanical stress and a fixed electric field, controlling the direction of application of stress and potential so that the scalar products of field and piezoelectric polarization have different values in the twinned portions of opposite sense; and insuring that the electrostatic stored energy difference, represented by the difference in scalar products, be larger than any countervailing energy difference either in elastically stored energy or electrostatic energy 7 allocable to'polarization componentsinduced by the field itself while said crystal is cooled through the inversion temperature from high to low quartz.

'7. Apparatus for detwinning a quartz crystal slab in which Dauphin twinning is present, comprising means for heating said slab'above the inversion temperature of quartz, and means for producing a homogeneous unidirectional stress in said slab and for applying alsteady potential difference to said slab during cooling after heating. 1

8. Apparatus for detwinning a quartz piezoelectric plate having substantially parallel major surfaces, comprising means for applying uniformly distributed mechanical pressure to the said major surfaces of the piezoelectric plate, means for applying a constant electric potential to said plate simultaneously with application of the mechanical pressure,-and means including a heater and enclosing chamber for heating said plate above theinversion point temperature of quartz and cooling-said plate through said inversion point simultaneously with the application of said pressure and-theapplication of electric potential thereto.

'9. Apparatus for detwinning a quartz wafer, comprising means including electrodes for applying homogeneous unidirectional stress normal to the major faces of said wafer, means'for including said electrodes'for applying a unipolar electric field to'sald quartz wafer simultaneously with the application of stress thereto, and means including a heater andenclosing chamber for heating said quartz wafer above the'quartz inversion 'pointtemperature of 573.3" (3., and for cooling said-wafer through said inversion point simultaneously with the application of stress and electric field to said-wafer.

10. Apparatus for removing Dauphin twinhing-from a piezoelectric quartzcrystal-blank having substantially parallel planar faces, comprising means for producing a homogeneous unidirectional stress within said blank, means for applying a constant potential to said blank, means for heating said blank beyond the temperature of inversion.fromlow-quartz tohighquartz, and means forpermitting cooling through thattemperaturewhile maintaining the application of potential and stress.

11. Apparatus for detwinning a quartz crystal slab, comprisingmeans for applying homogeneous unidirectional pressure to the major faces of said crystal slab, means for applying a constant potential difference between the faces of said crystal slab, means for heating said slab above the inversiontemperature of quartz, and means for cooling said slab slowly below said .temperature while maintaining pressure and potential difference thereon.

12. Apparatus for detwinning a quartz crystal slab, comprisingmeans for setting up a homogeneous unidirectional stress throughout said slab, means for applying a potential difference across said slab which will be the same at all points, and means for heating said slab above the inversion pointfrom low quartz to high quartz and permitting said slab to cool slowly while maintaining compressive stress and potential difierence thereacross.

13. Apparatus for detwinning a quartz slab having. substantially planar and parellel major faces and in which Dauphin twinning is present, comprising means for applying homogeneous unidirectional pressure to the major faces of said slab, means for applying a unipolar potential across said major faces, means for heating said slab abovethe inversion temperature. from low quartzto. high quartz, and means forcooling said. slab throughsaid inversion temperature while applying. such pressure and poten tial thereto.

ALAN N. HOLDEN.

REFERENCES CITED The following references are of record in the file of this patent:

Frondel, American Mineralogist, volume 30, Nos. 5 and 6, pages 447 to 460, 1945. George Banta Publishing v.00., Menasha, Wisconsin. (Copy in U. S. Geological Survey Library.) 

